[Clascomment] OPT-IN: First Observation of the Line Shape of the Lambda(1405) in Electroproduction

[Reinhard Schumacher]

Hello Volker,
	Here is the reply to your comments.  We have posted a new version of 
the paper (v2) on the CLAS reviews web site.  Many changes were made 
since the first draft, including in response to your particular concern.

Sincerely,
Reinhard and Haiyun

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On 06/09/2013 10:37 PM, Volker Burkert wrote:
> Dear lead authors,
> This is an interesting paper with potentially far reaching conclusions. We want to be sure
> that the conclusions are supported by the data and the analysis. Dan Carman made already
> many comments in this directions, many of which I agree with. I just wanted to add one
> more concern related to Figure 8. The conclusion from the figure adds really new
> information beyond what the previous paper on the Lambda(1405) line  shape already
> found, i.e. the complex structure of this resonance. The main message of this paper
> is that the higher mass structure of the two structures behave differently with Q2 than
> the lower mass structure and appears to show a harder form factor behavior, similar
> to the Lambda(1520). There could be interesting consequences in terms of underlying
> resonance structures if this is indeed the case.  Unfortunately, the current analysis
> leaves some questions to be addressed that I briefly discuss here:
>

We have replied to Dan's comments, and they are posted.

> Resonances are characterized by fixed masses and fixed widths. These are independent
> of Q2. However, the fit to the data at different Q2 ranges uses Breit-Wigner masses and
> widths, which apparently were left open to be fit in each Q2 range, separately. This leads
> to a changing first BW peak position from 1350MeV (at low average Q2) to 1365MeV (at high
> average Q2) as well as a changing width, and for the 2nd peak position a change from
> 1400 MeV (at low average Q2) to1420 MeV (at high average Q2) and a width changing
> from ~60MeV at the lowest average Q2 to  ~20MeV at the highest average Q2.
> This variation in fit parameters shows that some contribution is missing in the fit.
> In order to come to conclusions that are more supported by the data the fits should be
> done with fixed masses and widths for the Breit-Wigner forms and these parameters
> must be independent of Q2. Only then can the two BW amplitudes be compared and
> more definite conclusions drawn regarding the Q2 dependence.
>

We agree.  The fits shown in Figure 8 were repeated using only a single 
centroid and width for each of the two BW line shapes. Qualitatively the 
fits actually look better than before, albeit with slightly less good 
chi^2/d.o.f.  The mass centroids are in very good agreement with the fit 
shown in Fig 7.  We discuss this in the revised version of the paper. 
You'll note that we don't quote the widths or magnitudes of the line 
shapes, only the centroids.  This is because we think we are already at 
the limit of what we can reasonably claim in terms of accuracy of these 
results.

There is limit to how much we can get out of this data in terms of 
physics results.  We are trying not to make the concluding statements 
stronger than what the data support.  Clearly this work motivates 
getting higher statistics data in the future.

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> Volker
>